Reapply r237520 with another fix for infinite looping

SimplifyDemandedBits was "simplifying" a constant by removing just sign bits.
This caused a canonicalization race between different parts of instcombine.

Fix and regression test added - third time lucky?

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@237539 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Transforms/InstCombine/InstCombineCasts.cpp

@@ -435,6 +435,15 @@ Instruction *InstCombiner::visitTrunc(TruncInst &CI) {
   if (Instruction *Result = commonCastTransforms(CI))
     return Result;
 
+  // Test if the trunc is the user of a select which is part of a
+  // minimum or maximum operation. If so, don't do any more simplification.
+  // Even simplifying demanded bits can break the canonical form of a 
+  // min/max.
+  Value *LHS, *RHS;
+  if (SelectInst *SI = dyn_cast<SelectInst>(CI.getOperand(0)))
+    if (matchSelectPattern(SI, LHS, RHS) != SPF_UNKNOWN)
+      return nullptr;
+  
   // See if we can simplify any instructions used by the input whose sole
   // purpose is to compute bits we don't care about.
   if (SimplifyDemandedInstructionBits(CI))